Lack of ant attendance may induce compensatory plant growth

Three levels in ant-plant protection systems need to be considered to fully understand how these symbiotic systems work. Here we present the effect of Oecophylla smaragdina ants on (1) the arthropod community, (2) herbivory, and (3) plant performance, within a studied mangrove ant-plant protection system. On Rhizophora mucronata trees in Thailand ants successfully colonised ant trees attached with a string to a natural ant tree, whereas they were unable to colonise control trees without this connection. Trees were monitored and arthropods (numbers and composition), leaf damage, leaf turnover and growth rates (stem diameter, tree height and total leaf area) were recorded in two surveys covering a period of 12 months. The number of herbivorous arthropods, but not the number of predators, was significantly lower on ant trees compared to control trees. Likewise, the amount of leaf damage inflicted by the four major groups of herbivores (Chrysomelidae, Tortricidae, Geometridae and Sesarminae) was significantly lower on ant trees compared to control trees and so was the leaf turnover rate. In spite of this, the released herbivore pressure on ant trees did not translate into higher growth rates. In contrast, all growth responses increased more on control trees compared to ant trees. Differences between the two groups were insignificant but leaf area increase was only marginally nonsignificant (P=0.062). The results show that ants remove herbivorous arthropods more efficiently than predators but ant-colonised mangroves do not necessarily benefit from this despite the resulting decrease in herbivory.